Boosters of this kind are well known in the field of motor vehicles and in the conventional way comprise a casing having an axis of symmetry and divided in leaktight fashion by at least one moving wall structure into a front chamber permanently connected to a low-pressure source and a rear chamber selectively connected to the front chamber or to a high-pressure source by a three-way valve means actuated by an operating rod secured to a plunger, subjected to the action of a return spring and capable of sliding in a bore formed in a pneumatic piston, secured to the moving wall and a tubular rear part of which can slide in leaktight fashion in an opening in the rear wall of the casing, the three-way valve comprising a valve element arranged in the tubular rear part of the moving wall and interacting with a first valve seat formed on the plunger and with a second valve seat formed on the pneumatic piston, the rear position of rest of the plunger being defined by a stop formed at the front end of a sleeve arranged in the tubular rear part of the pneumatic piston.
A great many documents illustrate this type of booster, these including, for example, the document EP-A-0,233,026. For a long time attempts have been made to improve the performance of such boosters, and in particular to reduce the attack force and the response time so as to obtain rapid actuation of the brakes when the brake pedal is operated, and also to reduce the return time so that the driver can precisely gage his braking action.
The improvement in the response time can conventionally be obtained by increasing the cross section through which air can pass at the first annular valve seat, formed on the plunger, generally by increasing its diameter. By contrast, if the return time of the booster is to be reduced, that necessitates reducing the diameter of the second annular valve seat formed on the pneumatic piston. These two improvements are thus contradictory because, in this design of booster, the valve seats are located in more or less the same plane and the diameter of the first valve seat is limited, among other things, by the diameter of the second valve seat.